1. Technical Field
The present invention relates to a light-emitting element, a light-emitting device, a display device, and an electronic apparatus.
2. Related Art
An organic electroluminescence element (so-called organic EL element) is a light-emitting element having a structure in which a luminescent organic layer of at least one layer is interposed between an anode and a cathode. In such a light-emitting element, when an electric field is applied between a cathode and an anode, electrons are injected into a light-emitting layer from the cathode and holes are injected into the light-emitting layer from the anode, and thus the electrons and holes are recombined in the light-emitting layer, that is, carriers are recombined to generate excitons. When these excitons return to a ground state, the energy content is emitted as light.
Generally, such a light-emitting layer emitting light is formed of a luminescent material (luminescent dopant) and a host material.
Here, the host material, among the constituents of the light-emitting layer, is generally contained at the highest ratio. This host material serves to support the film of the light-emitting layer, and is configured such that, when a voltage is applied between electrodes, carriers are recombined in the molecule of the host material to generate excitation energy, and this excitation energy is transferred to a luminescent material, thereby causing the luminescent material to emit light. In contrast, the luminescent material is a compound having fluorescence or phosphorescence, and is configured such that it receives excitation energy from the host material to be excited, thus substantially emitting light.
In an organic El element having such a configuration, the recombination sites of carriers are determined by the carrier transportability of the host material used in the light-emitting layer. When a fluorescent material is used as the luminescent material, in many cases where a host material containing an acene-based compound such as anthracene or naphthacene is generally used as the host material thereof, bias is large in the carrier transportability of holes and electrons. Therefore, the recombination sites of carriers are concentrated in the vicinity of the interface of the light-emitting layer. As a result, only the luminescent material in the vicinity of the interface of the light-emitting layer, that is, only some luminescent material in the light-emitting layer contributes to light emission. Accordingly, there is a problem that, in the luminescent material located in the vicinity of the interface, the degradation of the luminescent material is locally promoted, and thus the deterioration of luminance of the light-emitting layer is accelerated.
In contrast to this, in recent years, it has been reported that a light-emitting layer contains an assist dopant material for controlling carrier transportability as a constituent material in addition to a luminescent material and a host material, and thus the carrier transportability in the light-emitting layer is controlled, thereby enhancing the efficiency of an organic EL element and increasing the lifetime thereof (for example, refer to JP-A-2005-108727).
However, in JP-A-2005-108727, the characteristics of the organic EL element are improved by controlling the relationship between HOMO and LUMO of the host material and the assist dopant material, but carrier transportability of holes and electrons cannot be sufficiently controlled only by controlling the relationship between HOMO and LUMO, and it cannot be said that the recombination sites of carriers in the light-emitting layer are set to a good position.